The reset mechanism of a thermal overload relay generally comprises a reset rod loaded pushably in a case, and by pushing said reset rod, a reversal mechanism performing a reversal operation accompanying the relay tripping returns to the initial state. This reset mechanism comprises a manual reset in which an operation of pushing in the reset rod is performed upon each reset, and an automatic reset in which the reversal mechanism is automatically returned to the initial state after cooling the bimetal member by holding the reset rod in the pushed-in state; the manual reset and the automatic reset being configured to be switchable.
FIG. 12 to FIG. 15 show a conventional thermal overload relay which is switchable between manual reset and automatic reset (see for example Patent Reference 1).
As shown in FIG. 12, this thermal overload relay comprises a bimetal member 2 which undergoes curving displacement due to heat generated by current conduction, and contact points 5 and 6 which cause the reversal mechanism 4 to perform a reversal operation and switch when the displacement position of the bimetal member 2 exceeds a stipulated value.
When the bimetal member 2 curves, displacement occurs in the right direction in FIG. 12, and this movement is transmitted via the shifter 8 to the release lever 9; the release lever 9 rotates in the counterclockwise direction with the shaft 10 as fulcrum. On the other hand, one end of a movable plate 14 as a fulcrum abuts against a V groove 11a on one end of a support piece 11 fixed to a case 1, and a tension spring 13 is hung across another end of the movable plate 14 and another end 11b of the support piece 11. And, a reversal plate 12 is fastened to the movable plate 14.
In the initial state of FIG. 12, the spring force from the tension spring 13 acts to rotate the reversal plate 12 in the clockwise direction, and the reversal plate 12 abuts and is halted in the state shown. In this initial state, a fixed constant point 5b of a normally closed contact 5 is mounted on the tip of a fixed contact point leaf spring 5a cantilever-supported by the case 1; this fixed contact point 5b contacts a movable contact point 5c mounted on the reversal plate 12. In addition, a fixed contact point 6b of a normally open contact 6 is mounted on the tip of a fixed contact point leaf spring 6a cantilever-supported in the proximity of the upper face of the case 1; and a movable contact point 6d is mounted on the tip of a movable contact point leaf spring 6c cantilever-supported substantially parallel to the fixed contact point leaf spring 6a, to oppose the fixed contact point leaf spring 6a. 
When the bimetal member 2 curves and is displaced due to heat generated by a passing current, the release lever 9 is rotated in the counterclockwise direction, the rotation of this release lever 9 rotates the tension spring 13 and reversal plate 12 in the counterclockwise direction, and as shown in FIG. 13, the normally closed contact 5 (5b, 5c) is opened, and the normally open contact 6 (6b, 6d) is closed, to enter the tripped state. The release lever 9, reversal plate 12, tension spring 13, normally closed contact 5 and normally open contact 6 constitute the reversal mechanism 4.
When the thermal overload relay enters the tripped state and the current of the electromagnetic contactor is shut off, the bimetal member 2 cools and returns to its initial state. However, the reversal mechanism 4 which has been reversed does not return to the initial state if a reset operation is not applied. Hence a reset rod 16 is provided to protrude from the upper face of the case 1.
As shown in FIG. 15, the reset rod 16 is a cylindrical member with a step comprising a large-diameter head portion 16a and a small-diameter shaft portion 16b, and as shown in FIG. 16, the reset rod 16 is mounted in a reset rod holding hole 3 provided in the case 1 to be slidable in the shaft direction and also rotatable. The reset rod holding hole 3 comprises a large-diameter hole portion 3a into the interior of which the large-diameter head portion 16a of the reset rod 16 is pushed, and a small-diameter hole portion 3b formed concentrically with this large-diameter hole portion 3a, and which slidably holds the small-diameter shaft portion 16b. 
In the upper face of the large-diameter head portion 16a is provided a groove 16c into which can be inserted a flat-blade screwdriver or other tool to rotate the reset rod 16. Further, on the small-diameter shaft portion 16b is provided an engaging piece 16d to protrude elastically, and in the tip at a position shifted 90° with respect to this engaging piece 16d is formed, by means of an inclined face and a vertical face, a cutout portion 16e cut out in an obtuse-angle shape. And as shown in FIG. 12, a leaf spring 6e integrated with the above-described fixed contact point leaf spring 6a abuts the cutout portion 16e of the reset rod 16.
The reset rod 16 loaded into the reset rod holding hole 3 is urged in the direction of protrusion from the case 1 by a return spring 7 comprising a compression spring inserted into the small-diameter shaft portion 16b; in FIG. 12 and FIG. 13 the reset rod 16 is in the manual reset position, and the reset rod 16 receiving the spring force of the return spring 7 is positioned in the axial direction by the engagement of the engaging piece 16d with a step portion 1a of the case 1 as shown in FIG. 12, so that the head portion protrudes from the display cover 18 occluding the upper face of the case 1. In the tripped state of FIG. 13, when an operation to push in the reset rod 16 is performed, the inclined face of the cutout portion 16e presses the leaf spring 6e, which is integral with the fixed constant point leaf spring 6a, from the cutout portion 16e. By this means the fixed contact point leaf spring 6a curves in the rightward direction, and presses the movable plate 14 to the right via the movable contact point leaf spring 6c. As a result, the reversal plate 12 in the reversed state is driven in clockwise rotation, and when the action of the tension spring 13 passes a dead point, the reversal plate 12 is reversed and returned to the initial state.
Next, in order to move from the manual reset position of FIG. 12 to the automatic reset position of FIG. 14, the tip of a flat-blade screwdriver or other tool is inserted into the groove 16c in the reset rod 16, and after pushing in the reset rod 16 until abutment occurs, the reset rod 16 is rotated 90° in the clockwise direction in FIG. 12. By this means, the reset rod 16 receiving the spring force of the return spring 7 from the upward axial direction is held in the pushed-in state while the engaging piece 16d engages the step portion 1b of the case 1 and is positioned in the axial direction. In this state, the tip of the leaf spring 6e which is integral with the fixed contact point leaf spring 6a is pressed out from the cutout portion 16e of the reset rod 16, and enters a state of riding up on the small-diameter shaft portion 16b of the reset rod 16. By this means, even in the initial state (non-reversed state) of FIG. 14, the gap between the fixed and movable contact points 6b, 6d of the normally open contact 6 is reduced. As a result, the passed current exceeds a stipulated value, and even when the reversal mechanism 4 begins a reversal operation, the movable contact point 6d does not contact the fixed contact point 6b and effect complete reversal before the reversal plate 12 completes reversal. Hence when the bimetal member 2 cools, the reversal mechanism 4 automatically returns to the initial state.
Patent Reference 1: Japanese Patent Publication No. 4088815